1. Field of the Invention
The present invention relates to an interference signal removing apparatus for removing narrow-band interference signals from input signals including wide-band desired signals and the narrow-band interference signals, and particularly to an interference-signal removing apparatus improved in suppressing that up to a desired signal is removed.
2. Description of the Prior Art
For example, reception signals received by a receiver may include a signal that should be received (desired signal) and a signal interfering with the desired signal (interference signal).
First, a wide-band desired signal and a narrow-band interference signal are described below by using radio LAN of IEEE 802.11 as an example.
The terms “wide band” and “narrow band” are used as relative meanings. Specifically, a signal having a sufficiently-large occupying band width compared to the occupying band width of a narrow-band interference signal is referred to as a wide-band signal and a signal having an occupying band width 10 times larger than the occupying band width of, for example, a narrow-band interference signal is referred to as a wide-band signal. As an example, in the case of the radio LAN described below, a wide-band signal has an occupying band with of, for example, 26 MHz (frequency per wave) and a narrow-band signal has an occupying band width of, for example, 2 MHz (frequency per wave).
The radio LAN of IEEE 802.11 roughly uses a direct diffusion (DSSS: Direct Sequence Spread Spectrum) system and a frequency hopping (FHSS: Frequency Hopping Spread Spectrum) system. In accordance with the difference between these modulated waves, it is possible to regard a signal according to the DSSS mode as a wide-band signal and a signal according to the FHSS mode as a narrow-band signal. Moreover, both systems perform radio communication by using the same frequency band and systematically allow mutual interference. Therefore, it is a matter of course that interference occurs between signals according to both systems.
In this case, the DSSS mode is a system for communicating (transmitting) a narrow-band signal as a wide-band signal through frequency diffusion and returning the signal to the original narrow-band signal in the demodulation process at the reception side. Therefore, the DSSS mode makes it possible to suppress a narrow-band signal included in a reception signal because the narrow-band signal is diffused to a wide-band signal in the demodulation process. A ratio before or after the above diffusion is referred to as a diffusion coefficient. For example, when the diffusion coefficient is equal to 128, a gain of approx. 21 dB (accurately, 10LOG128) is obtained.
The FHSS mode is a system for communicating a narrow-band signal by changing transmission frequencies of the signal every specific time and thereby using a wide band. Therefore, the FHSS mode makes it possible to suppress the influence of interference by a reception filter of a receiver using the FHSS mode because an occupying band width when fixing a specific time becomes as narrow as 2 MHz and the power per band concerned according to the DSSS mode relatively decreases.
Moreover, in the case of the FHSS mode, even while another transmitter communicates a signal in accordance with the FHSS mode using a different hopping pattern, the probability of using the same frequency at the same time is low. Therefore, the interference between the transmitters does not almost matter. Moreover, the FHSS mode allows frequency hopping by using a wide band compared to the case of the DSSS mode. Therefore, even if a strong interference occurs in the DSSS mode, it is possible to receive a signal in accordance with a frequency band free from interference.
In the case of the DSSS mode, however, a diffusion coefficient may be lowered in order to raise a signal transmission rate. Specifically, when the diffusion coefficient is lowered to 11, a gain is lowered approx. 10 dB (accurately, 10LOG11), moreover the gain is further lowered when the diffusion coefficient is lowered to less than 11, and the suppression effect of an interference signal may not be obtained.
Moreover, the probability that a signal according to the DSSS mode receives interference is raised because it has been started to widely use a standard such as Bluetooth (short-range mobile service) using, for example, the FHSS mode as the radio interface between portable units.
Furthermore, as another example, it is considered that an interference in an adjacent frequency band occurs between a communication signal according to the W-CDMA (Wideband-Code Division Multiple Access) mode and a communication signal according to the PHS (Personal Handyphone System) mode, an interference occurs between a wide-band signal of 2.4-GHz-band radio LAN (IEEE 80.2 11) and a narrow-band signal of Bluetooth, or an interference occurs between a CDMA-mode communication signal and a TDMA(Time Division Multiple Access)-mode or FDMA (Frequency Division Multiple Access)-mode communication signal due to common use of a frequency band or an interference with an unexpected external wave.
As an art for removing the above interference, the following methods have been studied so far: an interference-signal removing method using an adaptive algorithm and an interference-signal moving method using a notch filter. As an example, an art for removing a narrow-band signal interfering with a wide-band signal by a notch filter using a multi-rate filter bank is described in “Application of complex multi-rate filter bank to DS-CDMA/TDMA-signal bundling receiver sharing frequency band (Thesis journal of Institute of Electronics, Information, and Communication Engineers B-11, Vol. J80-B11, No. 12, December, 1997)”. However, this art also removes the component of a wide-band signal that is a desired wave when removing a narrow-band signal by a filter. Therefore, a problem occurs that a bit error ratio after removing an interference wave is deteriorated.
Next, a conventional interference-signal removing apparatus is described below. The interference-signal removing apparatus is set to, for example, a receiver for performing radio communication to remove an interference signal included in a signal received from the receiver.
FIG. 8 shows an interference-signal removing apparatus which is provided with an interference-signal extraction section 51, a synthesizer 52, and an interference-signal estimation section 53. In this case, symbol t denotes time.
The interference-signal estimation section 53 inputs a reception signal r(t) in which a wide-band desired signal and a plurality of narrow-band interference signals are synthesized and a reception signal e(t) after interference is removed, estimates an interference signal included in the reception signal r(t) by using a general adaptive algorithm, and outputs an interference-signal estimation coefficient h(t+1) according to the estimation result to the interference-signal extraction section 51.
The interference-signal extraction section 51 inputs a reception signal r(t), extracts a signal V(t) regarded as an interference signal from the reception signal r(t) in accordance with the interference-signal estimation coefficient h(t+1) input from the interference-signal estimation section 53, and outputs the interference signal V(t) to the synthesizer 52.
The synthesizer 52 synthesizes the reception signal r(t) with the interference signal V(t) output from the interference-signal extraction section 51 in an opposite phase (that is, so that the interference signal V(t) is removed from the reception signal r(t)) and outputs a reception signal e(t) in which the interference signal V(t) is removed. A part of the reception signal e(t) from which interference is removed output from the synthesizer 52 is input to the interference-signal estimation section 53 and used to estimate an interference signal.
Next, examples of CDMA mode and interference-signal removing apparatuses according to the CDMA mode are described below.
For example, a mobile communication system using the DS-CDMA mode realizes multiplex communication between a plurality of mobile station systems and a base station system by assigning different diffusion codes to the mobile station systems. Specifically, each mobile-station system diffuses, modulates, and transmits a signal to be transmitted by a diffusion code assigned to its own system while the base-station system demodulates a signal sent from a desired mobile-station system by inversely diffusing a reception signal by using a diffusion code assigned to each mobile-station system. Moreover, a mobile-station system demodulates a signal addressed to its own system by inversely diffusing a signal received from the base-station system by a diffusion code assigned to its own system.
FIG. 9 shows a diffusion-code series constituted of, for example, a PN (pseudo-noise signal) series.
As shown in FIG. 9, a diffusion signal of one unit (for one symbol) is constituted of a plurality of chip data values (e.g. string of values “1” and “−1”) and it is possible to generate a plurality of different diffusion codes by making patterns of chip-data-value strings different from each other. In this case, a diffusion code has a characteristic that by shifting a certain diffusion code up to one chip time or more, the correlation with this diffusion code disappears.
Moreover, FIG. 9 shows the time width of one chip data (chip interval Tc) and the time width of a diffusion code for one symbol (bit interval T). In this case, the time width of a diffusion code for one symbol corresponds to the time width of transmission data (e.g. values “1” and “0”) to be transmitted to a receiver (e.g. base-station system or mobile-station system) from a transmitter (e.g. mobile-station system or base-station system). That is, the change speed of chip data constituting a diffusion code is very high compared to the switching speed (symbol switching speed) of transmission data to be diffused and modulated by the diffusion code.
As described above, in the case of this type of radio communication, a different narrow-band signal (that is, signal according a mode other than the CDMA mode) is unexpectedly added into a wide frequency band used for the communication for which use of a frequency is permitted to cause an interference in some cases. When the above interference signal is larger than the degree of a disturbance due to noises estimated when a system is designed, the number of bit errors increases and the reception quality of a receiver is extremely deteriorated.
Moreover, as described above, it is also considered to realize multiplex communication in accordance with a mode for performing communication by using a comparatively-wide frequency band such as the CDMA mode and a mode for performing communication by using a narrow band such as the FM (Frequency Modulation) mode in order to effective use a frequency band. Specifically, it is principally possible to effectively use a frequency band by multiplexing a signal according to the analog communication mode such as the FM mode to the frequency band of a diffusion signal according to the CDMA mode. However, if a CDMA receiver cannot remove a signal according to the FM mode from a reception signal, the signal interferes with a diffusion signal and thereby, the number of bit errors increases and the reception quality is deteriorated.
FIG. 10 shows spectrums of reception signals including diffusion signals according to the CDMA mode (CDMA signal) and a signal according to the FM mode (FM interference wave), in which the abscissa indicates frequency and the ordinate indicates spectrum intensity.
The interference-signal removing apparatus (interference removing circuit) disclosed in the official gazette of Japanese Patent Application No. 11-197296 is described below by referring to FIGS. 11 to 15. The interference-signal removing apparatus disclosed in the official gazette is set to a base-station system, a mobile-station system, or a relay-station system using the CDMA mode to remove narrow-band interference signals from reception signals including wide-band diffusion signals diffused and modulated in accordance with the CDMA mode and the narrow-band interference signals or I and Q components of the reception signal, particularly removes the interference signal by using the characteristic of the diffusion signal.
FIG. 11 shows an interference-signal removing apparatus for removing an FM signal (interference signal) from the input signal r(t) by inputting reception signals including a CDMA signal (desired signal) and the FM signal. In the case of the interference-signal removing apparatus, to remove interference signals from reception signals including diffusion signals diffused and modulated in accordance with the CDMA mode and the interference signals, time-difference means 61 gives a time difference for one chip of a diffusion code or more between two signals obtained by distributing reception signals, extraction means 62 and 64 extract signal components having a correlation between the two signals provided with the time difference as interference-signal components, and removal means 63 removes the extracted interference-signal components from the reception signal.
Specifically, the interference removing apparatus shown in FIG. 11 is provided with a delay element 61 for delaying a reception signal, an adaptive filter 62 for extracting interference-signal components from the delayed reception signal in accordance with a tap-coefficient control signal output from a filter-tap-operation control section 64 to be described later, a subtracter 63 for removing the interference-signal components from the reception signal, and a filter-tap-coefficient-operation control section 64 for outputting a tap-coefficient control signal according to a signal output from the subtracter 63 and the delayed reception signal to the adaptive filter 62.
The configuration and operations of the circuit shown in FIG. 11 are described below.
The signal r(t) received from the receiver is input to the circuit, which includes a diffusion signal diffused and modulated in accordance with the CDMA mode and an interference signal (such as an FM signal) according to a communication mode using a narrow band. In this case, t indicates time, which is assumed as an integral discrete value using one sample time as the minimum unit in the case of this embodiment.
The above input signal r(t) is first divided into two signals, and one signal is input to the delay element 61 and the other signal is input to the subtracter 63.
The delay element 61 has a function for delaying an input signal for the time width for one chip of a diffusion signal or more and outputting the signal. The time difference is preset to a value capable of eliminating the correlative component of the diffusion signal between the above two signals and leaving the correlative component of an interference signal to be removed.
Specifically, a signal output from the delay element 61 is shown as r(t−τ). In this case, τ denotes a delay time output from the delay element 61.
The signal (t−τ) output from the delay element 61 is input to the adaptive filter 62 and filter-tap-coefficient-operation control section 64.
FIG. 12 shows a configuration of the adaptive filter 62.
The adaptive filter 62 shown in FIG. 12 is provided with a shift register constituted of (n−1) storage elements S1 to Sn-1 arranged in series, n multipliers J1 to Jn, and (n-1) adders K1 to Kn-1. In this case, symbol n denotes the number of filter taps.
A signal r(t−τ) output from the delay element 61 is input to the shift register and stored in the storage elements S1 to Sn-1 in time series. Moreover, a signal stored in each of the storage elements S1 to Sn-1 is successively shifted to the following storage element.
Specifically, the series u(t) of the signal r(t−τ) input to the shift register in the shift register is shown by the following expression (1). In the expression (1), u(t) denotes a vector.
In this specification, when a symbol used to show a signal or the like does not show a vector or matrix, the symbol is assumed as scalar.
[Numerical Formula 1]u(t)={r1, r2, r3, . . . , rn}rx=r(t−τ−x+1)  (Expression 1)
In this case, a signal r1 is a signal to be input to the shift register at a certain time and output to the multiplier J1 without passing through any one of the storage elements S1 to Sn-1. Moreover, signals r2 to rn are signals to be output from the storage elements S1 to Sn-1 at the time concerned and output to the multipliers J2 to Jn.
The above signals r1 to rn are input to the multipliers J1 and Jn and moreover tape-coefficient control signals h1 to hn are input to the multipliers J1 to Jn from a filter-tap-coefficient-operation control section 64 to be described later one to one. The multipliers J1 to Jn multiply two input signals (that is, weight the signals r1 to rn with the tap-coefficient control signals h1 to hn) and output the multiplication results to the adders K1 to Kn.
In this case, a filter-tap-coefficient series h(t) output from the filter-tap-coefficient-operation control section 64 is shown by the following expression (2). In this case, h(t) denotes a vector.
[Numerical Formula 2]h(t)={h1, h2, h3, . . . , hn}  (Expression 2)
Moreover, multiplication results output from the multipliers J1 to Jn are totaled by the adders K1 to Kn and the totaled result is output from the adaptive filter 62. In this case, as described later, the filter-tap-coefficient series h(t) of this embodiment is updated one after another by the filter-tap-coefficient-operation control section 64 so that the totaled result becomes the same signal as the interference-signal component included in a reception signal.
Specifically, a signal output from the adaptive filter 62 (that is, the above totaled result) FM(t) is shown by the following expression (3). In this case, Σ in the expression (3) denotes a sum.
[Numerical Formula 3]FM(t)=h(t)*u(t)=Σ(hi*ri)(i=1, 2, . . . , n)  (Expression 3)
A symbol “*” used for this specification denotes the multiplication between symbols arranged before and after the symbol “*”. Particularly, the multiplication between vectors shows the operation for calculating the inner product of two vectors.
As described above, the adaptive filter 62 extracts the above interference signal component from the input delay signal r(t−τ) in accordance with a tap-coefficient control signal output from the filter-tap-coefficient-operation control section 64 and outputs the component to the subtracter 63 as an interference-wave extraction signal FM(t).
The subtracter 63 has a function for inputting the not-delayed input signal r(t) and the signal FM(t) output from the adaptive filter 62, subtracting the output signal FM(t) from the input signal r(t), and outputting the subtraction result e(t).
In this case, the above subtraction result e(t) is a signal output from the interference-signal removing apparatus of this embodiment, which is shown by the following expression (4).
[Numerical Formula 4]e(t)=r(t)−FM(t)  (Expression 4)
In the case of this embodiment, tap-coefficient control signals output from the filter-tap-coefficient-operation control section 64 to be described later are updated one after another and thereby, the above interference-wave extraction signal FM(t) becomes the same signal as an interference signal in a reception signal. Therefore, the above subtraction result e(t) becomes a signal obtained by removing the interference signal from the reception signal, that is, a diffusion signal according to the CDMA mode (ideally, only the diffusion signal).
The filter-tap-coefficient-operation control section 64 has a function for receiving the signal r(t−τ) output from the delay element 61 and the signal e(t) output from the subtracter 63, computing a tap-coefficient in which the signal FM(t) output from the adaptive filter 62 becomes a signal same as an interference-signal component by using the signals r(t−τ) and e(t), and outputting the operated tap-coefficient control signal to the adaptive filter 62.
The filter-tap-coefficient-operation control section 64 of this embodiment can compute the above tap-coefficient control signal by using an algorithm such as LMS (Least Means Square) or RLS (Recursive Least Square). For this embodiment, a case of using an LMS algorithm is described and moreover, a case of using a RLS algorithm will be described later.
First, the general expression of LMS is described below.
The update expression of LMS is generally shown by the following expression 5.
[Numerical Formula 5]h(t+1)=h(t)+u*e(t)*u(t)  (Expression 5)
In the above expression, h(t) denotes a filter tap coefficient at the time t, μ denotes a step size parameter which is a coefficient about the time or accuracy of convergence, e(t) denotes an error signal at the time t, and u(t) denotes an input signal series at the time t.
Moreover, the above error signal e(t) is generally shown by the following expression (6).
[Numerical Formula 6]e(t)=d(t)−u(t)*h(t)  (Expression 6)
In this case, d(t) is generally referred to as a unique word or training signal, which uses an already-known signal predetermined at the transmission and reception sides. The operation algorithms used for the expressions (5) and (6) make it possible to converge the error signal e(t) to 0 by updating filter-tap-coefficient series one after another.
Then, a case is described below in which the above LSM algorithm is applied to this embodiment.
When applying the above expression (5) to this embodiment, h(t) serves as a filter-tape-coefficient series output from the filter-tap-coefficient-operation control section 64 to the adaptive filter 62 and u(t) serves as a signal series (shown in the above expression 1) to be output from the delay element 61 to the filter-tap-coefficient-operation control section 64.
Moreover, this embodiment uses a signal (shown in the above expression 4) output from the subtracter 63 as the above error signal e(t) and this is a feature of the interference removing circuit of this embodiment and the processing different from the normal LMS algorithm is performed.
First, if the delay element 61 is not used, the signal e(t) output from the subtracter 63 converges to 0 because the above operation algorithm makes the error signal e(t) approach to 0 and the filter-tap-coefficient series h(t) is generated which removes not only an interference signal but also a diffusion signal according to the CDMA mode from a reception signal.
Moreover, because this embodiment is provided with the above delay element 61, a time difference equal to a delay time τ is present between a signal r(t−τ) input from the delay element 61 to the filter-tap-coefficient-operation control section 64 and the signal e(t) input to the filter-tap-coefficient-operation control section 64 through the subtracter 63.
In this case, the diffusion signal r(t) according to the CDMA mode is not correlated with the diffusion signal r(t−τ) delayed by one-chip time or more from the signal r(t). Therefore, when the operation algorithm converges the error signal e(t) to 0, the diffusion signal component of u(t) is not correlated with r(t) and thereby, the component is left as the error e(t). That is, because the influence of the diffusion signal component theoretically becomes 0 by continuously adding the input signal series u(t) in the above expression (4), the diffusion signal component is not removed but it is left as the error e(t). However, because the interference signal component which temporally slowly fluctuates compared with chip data has a correlation even if there is a delay of several chip times, the filter-tap-coefficient series h(t) capable of removing only the interference signal component from a reception signal is generated.
That is, the above operation algorithm applied to this embodiment leaves a component (that is, interference signal component in which u(t) correlates with e(t) in a signal output from the adaptive filter 62 while the algorithm can generate the filter-tap-coefficient series h(t) which does not leave a component having no correlation (that is, diffusion signal component) in a signal output from the adaptive filter 62.
According to the above operation algorithm, the adaptive filter 62 of this embodiment can extract only the interference component from a reception signal and output the component to the subtracter 63. The subtracter 63 can output a signal obtained by removing only an interference signal component from a reception signal (that is, diffusion signal according to the CDMA mode).
As described above, the interference-signal removing apparatus shown in FIG. 11 makes it possible to adaptively remove narrow-band interference signals from reception signals including wide-band diffusion signals diffused and modulated in accordance with the CDMA mode and the narrow-band interference signals by using the characteristic of the diffusion signal, prevent reception quality from deteriorating, and improve the reception quality.
Though FIG. 11 shows a configuration for preventing a signal output from the subtracter 63 from being delayed, it is also possible to obtain the same advantage as the above mentioned by a configuration for delaying a reception signal input to the subtracter 73 by the delay element 71 while preventing a reception signal input to the adaptive filter 72 or filter-tap-coefficient-operation control section 74 from being delayed as shown in FIG. 13. The configuration shown in FIG. 13 is almost the same as the configuration shown in FIG. 11 except that the delay element 71 is set to the subtracter 73.
Moreover, it is possible to obtain the interference removal effect same as the above mentioned by using an algorithm other than the above LMS algorithm. For example, a specific update expression when using the RLS algorithm in the configuration shown in FIG. 11 is described below. In the description below, objects corresponding to the above u(t), h(t), e(t), d(t) and r(t) are shown by the same symbols for convenience' sake of description.
For example, an n-row one-column vector constituted of the same component as u(t) shown by the above expression 1 is assumed as an input series u(t) and an n-row one-column vector constituted of n filter tap coefficients similarly to h(t) shown in the above expression 2 is assumed as a filter-tap series h(t).
Moreover, the error signal e(t) in RLS is shown by the following expression 7 as a signal corresponding to the error signal e(t) shown by the above expression 6. Moreover, uT(t) shows transposed u(t).
[Numerical Formula 7]e(t)=d(t)−uT(t)*h(t)  (Expression 7)
In the case of this embodiment, the reception signal r(t) input to the subtracter 63 is used as d(t) and uT(t)*h(t) in the above expression 7 corresponds to an interference extraction signal output from the adaptive filter 62. That is, similarly to the case of using the above LMS algorithm, the error signal e(t) shown by the above expression 7 uses a signal output from the subtracter 63 and this is a feature of this embodiment.
When the delay element 61 is not used similarly to the case of using the above LMS algorithm, the error signal e(t) converges to 0.
Moreover, by using a coefficient-error correlation matrix P(t) that is an n-row n-column matrix and a gain vector k(t) that is an n-row one-column vector, the update expression of RLS is shown by the following expressions 8 to 10.
[Numerical Formula 8]h(t)=h(t−1)+k(t)*e(t)  (Expression 8)[Numerical Formula 9] k(t)={P(t−1)*u(t)}/{1+uT(t)*P(t−1)*u(t)}  (Expression 9)[Numerical Formula 10]P(t)=P(t−1)−k(t)*uT(t)*P(t−1)  (Expression 10)
Moreover, the initial value h(0) of the above filter-tap-coefficient series h(t) uses a zero vector as shown by the expression 11 and the initial value P(0) of the above coefficient-error correlation matrix P(t) uses a matrix in which every diagonal element in which the number of rows coincides with the number of columns is a positive real number c and elements other than the diagonal element are 0 as shown by the expression 12. Symbol hT(0) denotes transposed h(0). Moreover, I in the expression 12 denotes an n-row n-column matrix in which every diagonal element in which the number of rows coincides with the number of columns is 1 and elements other than the diagonal element are 0.
[Numerical Formula 11]hT(0)={0, 0, 0, . . . , 0}  (Expression 11)[Numerical Formula 12]                              P          ⁡                      (            0            )                          =                              c            *            I                    =                      (                                                            c0                                                  ⋯                                                  0                                                                              0                                                  ⋯                                                  ⋯                                                                              ⋯                                                  c                                                  ⋯                                                                              ⋯                                                  ⋯                                                  0                                                                              0                                                  ⋯                                                                      0                    ⁢                    c                                                                        )                                              (                  Expression          ⁢                                           ⁢          12                )            
A filter-tap-coefficient-operation control section 24 updates filter-tap-coefficient series h(t) one by one in accordance with the above update expression of RLS and thereby, it is possible to make a signal output from the adaptive filter 62 slowly approach to an actual interference signal component similarly to the case of using the above LMS algorithm. Thus, it is possible to remove narrow-band interference signals from reception signals including wide-band diffusion signals diffused and modulated in accordance with the CDMA mode and the narrow-band interference signals.
FIG. 14 shows an interference-signal removing apparatus for inputting I and Q components of reception signals including a CDMA signal (desired signal) and an FM signal (interference signal) and removing the FM signal from the I component rI(t) and the Q component rQ(t). In the case of the interference-signal removing apparatus, when removing the interference signals from the I and Q components of the reception signals including the diffusion signals diffused and modulated in accordance with the CDMA mode and the interference signals, time-difference means 81a and 81b provides a time difference for one chip of the diffusion signal between two signals obtained by dividing I component and between two signals obtained by dividing Q component, extraction means 82a, 82b, 83a, and 83b extract I and Q components from an interference signal component by using a signal component correlated between one reception signal constituted of I and Q components provided with the time difference and the other reception signal constituted of I and Q components as the interference signal component, and removal means 84a, 84b, 85a, and 85b remove I component of the extracted interference signal component from I component of the reception signal and Q component of the extracted interference signal component from Q component of the reception signal.
Specifically, the interference-signal removing apparatus shown in FIG. 14 is provided with a delay element 81a for delaying an I-phase signal (I component) orthogonally-detected from a reception signal, a delay element 81b for delaying a Q-phase signal (Q component) orthogonally-detected from the reception signal, four adaptive filters 82a, 82b, 83a, and 83b for extracting an interference signal component from I or Q component delayed in accordance with a tap-coefficient control signal output from a filter-tap-coefficient-operation control section 86 to be described later, an adder 84a for adding I component of the interference signal component, an adder 84b for adding Q component of the interference signal component, a subtracter 85a for removing I component of the interference signal component from I component of the reception signal, a subtracter 85b for removing Q component of the interference signal component from Q component of the reception signal, and a filter-tap-coefficient-operation control section 86 for outputting a tap-coefficient control signal according to signals output from the subtracters 85a and 85b and I and Q components of a delayed reception signal to the adaptive filters 82a, 82b, 83a, and 83b. 
A configuration and operations of the circuit shown in FIG. 14 are described below.
I component rI(t) and Q component rQ(t) orthogonally detected from a reception signal by a receiver are input to the circuit and the input signals rI(t) and rQ(t) include a wide-band diffusion signal diffused and modulated in accordance with the CDMA mode and an interference signal (e.g. FM signal) according to a communication mode using a narrow band. In this case, similarly to the case of performing description by referring to FIG. 11, t denotes time, which is assumed as an integral discrete value using one-sample time as the minimum unit in the case of this embodiment.
The above I component rI(t) is first divided into two signals in which one signal is input to the delay element 81a while the other signal is input to the subtracter 85a. Moreover, the above Q component rQ(t) is first divided into two signals in which one signal is input to the delay element 81b while the other signal is input to the subtracter 85b. 
Each of the delay elements 81a and 81b has a function for delaying an input signal by the time width for one chip of a diffusion signal or more and outputting the signal, for example, similarly to the delay element 61 as that shown in FIG. 11. These two delay elements 81a and 81b provide the same delay time. Moreover, similarly to the case of performing description by referring to FIG. 11, specifically, the signal of I component output from the delay element 81a is shown as rI(t−τ) and the signal of Q component output from the delay element 81b is shown as rQ(t−τ). In this case, T denotes a delay time provided from the delay elements 81a and 81b. 
The signal rI(t−τ) output from the delay element 81a is input to two adaptive filters 82a and 83b and the filter-tap-coefficient-operation control section 86 while the signal rQ(t−τ) output from the delay element 81b is input to two adaptive filters 82b and 83b and the filter-tap-coefficient-operation control section 86.
The configuration of each of the adaptive filters 82a, 82b, 83a, and 83b is the same as that shown in FIG. 12. In this case, this embodiment is provided with four adaptive filters 82a, 82b, 83a, and 83b in order to perform the complex operation of I and Q phases. Specifically, this is because I and Q components of an interference signal component are included in I and Q components of a reception signal. Moreover, this embodiment uses two filter-tap-coefficient series hI(t) and hQ(t) of I and Q phases. In this case, hI(t) and hQ(t) are vectors.
Specifically, in the case of this embodiment, filter-tap-coefficient series hI(t) and hQ(t) for making it possible that the adaptive filter 82a can extract I component of an interference signal component from I component rI(t−τ) of an input reception signal, the adaptive filter 83a can extract Q component of the interference signal component from I component rI(t−τ) of the input reception signal, the adaptive filter 82b can extract Q component of the interference signal component from Q component rQ(t−τ) of the input reception signal, and the adaptive filter 83b can extract I component of the interference signal component from Q component rQ(t−τ) of the input reception signal are generated by the filter-tap-coefficient-operation control section 86 to be described later.
The adder 84a has a function for adding signals output from the adaptive filters 82a and 83b and outputting the addition result to the subtracter 85a. The addition result output to the subtracter 85a becomes the interference signal component in I component of a reception signal (that is, I component of the interference signal component) FMI(t). In the case of this embodiment, the adder 84a inverts positive and negative of a signal output from the adaptive filter 83b and performs the above addition. However, when inversion of positive and negative is performed by the above adaptive filter 83b or the filter-tap-coefficient-operation control section 86 to be described later, the adder 84a does not have to perform the above inversion of positive and negative.
The adder 84b has a function for adding signals output from the adaptive filters 82b and 83b and outputting the addition result to the subtracter 85b. The addition result output to the subtracter 85b becomes an interference signal component in Q component of a reception signal (that is, Q component of interference signal component) FMQ(t).
In this case, I component FMI(t) of the interference signal component output from the above adder 84a is shown by the following expression 13 and Q component FMQ(t) of the interference signal component output from the above adder 84b is shown by the following expression 14. In the expressions 13 and 14, uI(t) and uQ(t) are vectors and correspond to I and Q components of u(t) shown by the expression 1 in the description using FIG. 11.
[Numerical Formula 13]FFMI(t)={hI(t)*uI(t)}+{−hQ(t)*uQ(t)}  ((Expression 13)[Numerical Formula 14]FFQ(t)={hI(t)*uQ(t)}+{hQ(t)*uI(t)}  (Expression 14)
The subtracter 85a has a function for inputting the input signal rI(t) of not-delayed I component and the signal FMI(t) output from the adder 85a, subtracting the output signal FMI(t) from the input signal rI(t), and outputting the subtraction result eQ(t).
Similarly, the subtracter 85b has a function for inputting the input signal rQ(t) of not-delayed Q component and the signal FMQ(t) output from the adder 84b, subtracting the output signal FMQ(t) from the input signal rQ(t), and outputting the subtraction result eQ(t).
In this case, the above subtraction results eI(t) and eQ(t) are signals output from the interference-signal removing apparatus of this embodiment.
In the case of this embodiment, tap-coefficient control signals output from the filter-tap-coefficient-operation control section 86 to be described later are updated one by one and thereby, interference-wave extraction signals FMI(t) and FMQ(t) of the above I and Q components become the same signals as interference signals in I and Q components of a reception signal. Therefore, the above subtraction results eI(t) and eQ(t) become signals obtained by removing the interference signals from I and Q components of the reception signal, that is, diffusion signals according to the CDMA mode (ideally, only the diffusion signals).
The filter-tap-coefficient-operation control section 86 receives the signals rI(t−τ) and rQ(t−τ) from the delay elements 81a and 81b and the signals eI(t) and eQ(t) from the subtracters 85a and 85b. The filter-tap-coefficient-operation control section 86 has a function for computing tap-coefficient control signals for changing signals output from the adaptive filters 82a, 82b, 83a, and 83b to the above interference signal components, and outputting them to the adaptive filters 82a, 82b, 83a, and 83b. This embodiment is set so that the same tap-coefficient control signals are output to the adaptive filters 82a and 82b while the same tap-coefficient control signals are output to the remaining adaptive filters 83a and 83b and thereby, the interference signal components FMI(t) and FMQ(t) shown by the above expressions 13 and 14 are generated.
The filter-tap-coefficient-operation control section 86 of this embodiment computes a tap-coefficient control signal by using the algorithm for complex operation of LMS shown in the description using FIG. 11. The update expressions of LMS in this algorithm are shown by the following expressions 15 and 16.
[Numerical Formula 15]hI(t+1)=hI(t)+u*(eI(t)*uI(t)+eQ(t)*uQ(t))  (Expression 15)[Numerical Formula 16]hQ(t+1)=hQ(t)+u*(eQ(t)*uI(t)−eI(t)*uQ(t))  (Expression 16)
In the above expressions, hI(t) and hQ(t) denote filter-tap-coefficient series at the time t, μ denotes a step size parameter serving as a coefficient relating to convergence time and accuracy, and uI(t) and uQ(t) denote input signal series in shift registers of the adaptive filters 82a, 83a and shift registers in the adaptive filters 82b and 83b. Moreover, similarly to the case of performing description by referring to FIG. 11, eI(t) and eQ(t) use signals output from the subtracters 85a and 85b. uI(t) and uQ(t) denote vectors as described above.
Similarly to the case of performing description by referring to FIG. 11, by updating the filter-tap series hI(t) and hQ(t) by the operation algorithm sequentially this embodiment makes it possible to generate the filter-tap series hI(t) and hQ(t) capable of removing interference signal components having a comparatively high correlation each other without removing diffusion signal components because they are not correlated with each other.
Moreover, because this embodiment considers I and Q components when computing the filter-tap-coefficient series hI(t) and hQ(t), it is possible to further improve the accuracy of interference removal.
As described above, the interference-signal removing apparatus shown in FIG. 9 can remove interference signals from I and Q components of reception signals including diffusion signals diffused and modulated in accordance with the CDMA mode and the interference signals by using the characteristic of the diffusion signal and thereby, it is possible to prevent reception quality from deteriorating and improve the reception quality.
Similarly to the case of performing description by referring to FIG. 11, FIG. 14 shows a configuration for preventing signals output from the subtracters 85a and 85b from delaying. However, as shown in FIG. 15 it is possible to obtain the same advantage as the above by a configuration for delaying reception signals input to subtracters 95a and 95b by delay elements 91a and 91b while preventing reception signals input to adaptive filters 92a, 92b, 93a, and 93b and a filter-tap-coefficient-operation control section 96 from delaying. In this case, the configuration shown in FIG. 15 is the same as the configuration shown in FIG. 14 except that the delay elements 91a and 91b are provided for the subtracters 95a and 95b and adders 94a and 94b are also provided together with the above components.
Moreover, similarly to the case of performing description by referring to FIG. 11, it is possible to obtain the interference removal effect same as the above described by using an algorithm other than the above-described LMS algorithm for complex operation. As an example, the case of using the RLS algorithm for complex operation for the configuration shown in FIG. 9 is described below. For convenience' sake of description, objects corresponding to the above uI(t), uQ(t), hI(t), and hQ(t), eI(t), eQ(t), rI(t), and rQ(t) are provided with same symbols.
In the case of the RLS algorithm for complex operation, all parameters of u(t), h(t), e(t), k(t), and P(t) shown by the above expressions 7 to 10 are constituted of complex-number elements. In this case, when assuming γ and ω as real numbers and using j as a symbol for showing an imaginary-number part, an optional complex-number element is shown as (γ+jω).
Moreover, in the case of the RLS algorithm for complex operation, the sequential updating described by referring to FIG. 11 is realized in complex operation by separating the real-number part from the imaginary-number part of each of the above parameters and using the real-number part as an I-component parameter and the imaginary-number part as a Q-component parameter.
Specifically, in the case of this embodiment, the processing for removing interference signal components from the I component rI(t) and Q component rQ(t) of a reception signal is performed by assuming the real-number part of u(t) as uI(t) and the imaginary-number part of u(t) as uQ(t), the real-number part of h(t) as hI)(t) and the imaginary-number part of h(t) as hQ(t), and the real-number part of e(t) as eI(t) and the imaginary-number part of e(t) as eQ(t).
As described above, also when using the RLS algorithm for complex operation, it is possible to remove interference signals from I and Q components of reception signals including diffusion signals diffused and modulated in accordance with the CDMA mode and the interference signals similarly to the case of using the above LMS algorithm for complex operation.